Motion detectors using passive infrared (PIR) technology are widely used in the field of security. There are two key components in this type of detector, the one is a Fresnel lens array window which can focus infrared energy produced by a heat source (such as human body) onto a pyroelectric sensor that can convert the changes of infrared energy reaching it into an electrical signal; and the other is a pyroelectric sensor that can convert the infrared energy into an electrical signal. For example, if there is no motion heat source, then the sensor does not output characteristic signal (large amplitude changing randomly), and if there is a person walking in the monitoring area, then the sensor detects the temperature difference between the human body and the background, and output the corresponding characteristic signal.
The signals are amplified, sampled, and processed by hardware circuit and algorithms that determine whether there is an intruder or not, and a corresponding control output can be generated. Thus, in known PIR detectors, the movement of a heat source is sensed. Some detectors, by combining microwave technology with a PIR sensor, attempt to prevent false alarms generated by only using the PIR technology.
A block diagram of a known PIR-type detector is illustrated in FIG. 1. The detector of FIG. 1 includes a PIR sensor module 110, an analog signal processing module 120, a master controlling unit (MCU) module 130. In the detector of FIG. 1 the sensor module outputs electric signals in response to sensing the motion of a human body. The signals are amplified by the analog circuit, and are then processed to make a determination as to the presence of a moving body. An alarm indicating output signal can then be produced and forwarded to a monitoring system.
An alarm indicating output signal is generated if the signal amplitude is higher than the “high-threshold” or is lower than the “low-threshold” and persists for certain time. In response thereto, a PIR alarm indicating output signal is emitted.
The principle of signal processing by using this method is illustrated in FIG. 2. An output signal 210 from a PIR-type sensor varies about a signal baseline 220. A high-threshold 230 and a low-threshold 240 are pre-established.
Relative to the PIR signal as shown, if ΔT1>ΔT_TH or ΔT2>ΔT_TH (ΔT_TH is the pre-set time threshold), then the PIR detector is triggered, and an alarm indicating signal is emitted.
Disadvantages of the above described method include, missing alarms due to smaller output signals. Such signals might be generated, for example, by an intruder wearing protective clothing, thick clothes, or using an umbrella to block infrared emissions. In other circumstance, it is easy to trigger false alarms for burst signals, such as these signals generated by a sudden shock, a jarring, or a burst hardware inference.
In summary, in known PIR-type detectors, the output signals from PIR-type sensors are indicative of sensed movement of heat sources in the region being monitored. For example, intruder speed, height, weight, dress, behavior, posture, and temperature variations contribute to generating signal waveforms with complex characteristics which result in difficulty in making accurate alarm determinations. This in turn produces undesirable failures to properly emit alarm signals, or the emission of undesirable false alarms.